2D melting and motility induced phase separation in Active Brownian Hard Disks and Dumbbells

Abstract

Recently, we characterized the complete phase transition diagram in the ϕ-Pe parameter space for two models of active brownian particles in two dimensions. These models are composed of hard disks and dumbbells, respectively, the former being isotropic and the latter anisotropic. Here, we want to outline all the most significant features of these two paradigmatic models and compare them.Remarkably, the phase diagrams of the two models are affected differently by the introduction of activity. Disks present a two-stage melting scenario from Pe=0 to about Pe=3, with a first order phase transition between liquid and hexatic and a Berezinskii-Kosterlitz-Thouless transition between hexatic and solid. At higher activities, the three phases are still observed, but the transition between liquid and hexatic becomes a BKT transitions without a distinguishable coexistence region. Dumbbells, instead, present a macroscopic coexistence between hexatically ordered regions and disordered ones, over a finite interval of packing fractions, for all activities, included Pe=0, without any observable discontinuity in the behavior upon increasing Pe.